Bimetallic light metal piston



United States Patent Oice 2,801,890 Patented Aug. 6, 1957 BHWETALLIC LIGHT METAL PISTON Alfred Nitsch, Nurnberg, Germany, assigner to Aluminium Laboratories Limited, Montreal, Quebec, Canada, a company of Canada Application February 9, 1954, Serial No. 409,180 Claims priority, application Germany February 18, 1953 6 Claims. (Cl. 309-11) The invention relates to pistons made of light-metal, such as aluminium and aluminium and magnesium alloys, for internal combustion engines, compressors and the like of the type having a skirt which is partly separated from the piston head by peripheral horizontal slots or by a row of holes or perforations. It is known to incorporate inserts in the skirts of such pistons, the inserts taking the form of arcuate strips of a metal which has a thermal coelllcient of expansion which differs from that of the material of the piston, so that the inserts form with the skirt, bimetallically connected parts, so as to set up bending stresses in the skirt with changes of temperature. It has already been proposed to employ rnetallic inserts in order to obtain definite changes of shape of the piston when raised to its operating temperature. The object of the present invention is to control these changes of shape so that they are different at diierent levels in the skirt for a given temperature rise in order to be able to offset in a simple manner the changes of shape of the piston, which in operation are due to difierent temperatures at the different positions in the piston.

The object of the present invention is to control the effects of thermal expansion at different levels of the skirt by simple means, so that a continuous conformation of the piston to the cylinder wall is possible at all levels to allow the installation slackness in the cold condition to be reduced to a minimum. Any increase of weight by the inclusion of additional ribs is to be avoided and the weight of the inserts which consist of heavy metal is to be kept to a minimum.

According to the invention, a bimetallic effect, which may be altered in accordance with the requirements, is obtained at different levels of the skirt by virtue of the fact that the inserts are constructed so that their resistance to bending varies between their top and bottom edges. To eiect this, the thickness of the inserts and/or of the piston skirt may be continuously varied or progressively stepped between its edges. In practice, this can be effected in various ways. In a preferred embodiment, the variation of the bimetallic action at different levels of the skirt is elected by providing perforations or one or more rows of perforations in the inserts. The perforations may be of dilerent shapes and sizes. This embodiment gives the special advantage of a considerable saving in weight. The inserts may have imperforate zones next to the perforations or rows of perforations.

In another embodiment, the inserts have a slightly wedge-shaped crosssection. They may, instead, have a cross-section, the thickness of which is stepped.

Instead of influencing the shape of the skirt at the different levels by variation of the thickness or shape of the inserts a like effect may be obtained by varying the thickness of the piston skirt continuously in the region of the inserts, for example by constructing it with a slightly wedge-shaped cross-section or a stepped cross-section, where it is in contact with the inserts.

Finally, the two expediments may be employed together in order to obtain the desired result with regard to the variation of the expansion relationships at the different levels of the piston skirt.

The drawings show a few examples of embodiments of the present invention.

Figure 1 is a vertical section of a light-metal piston;

Figure 2 is a fragmentary vertical section in a plane at right-angles to that of Figure 1;

Figure 3 is a horizontal section on line III-III of Figure l;

Figure 4 is a development of an insert with two rows of circular holes;

Figure 5 is a development of an alternative insert with two rows of rectangular holes;

Figure 6 is a vertical section through an insert with a slightly wedge-shaped cross-section;

Figure 7 is a cross-section through an insert of stepped thickness, and

Figure 8 is a partial vertical section, displaced through 45 to the axis of the piston pin, through the skirt wall of a piston, having a skirt of wedge-shaped cross-section reinforced by a metallic insert.

In the embodiment shown in Figures 1 to 3, the piston head 1, which contains annular piston grooves 2, is separated from the piston shank 4 over parts of the periphery by transverse slots 3. The pin bosses 5 are connected, in the known manner, with the bottom of the piston head by stays 6. On the inside of the skirt 4 are inserts 7, in the form of sleeve segments, which have a thermal coeli'icient of expansion which differs from that of the material of the piston, so that the inserts 7 combine with the adjacent parts of the skirt 4 to form bimetallic members.

The inserts have apertures 11 for the piston pins. In addition, there are, in the bottom part of the insert, three rows of perforations 8, 9 and 10 of different sizes. In this way, the resistance to bending of the insert 7 decreases from the top towards the bottom of the insert in a definite manner, which can be controlled as desired by varying the size, the number and the arrangement of the perforations.

Figures 4 and 5 show the developments of diiierently constructed inserts, which may be substituted for the insert 7 shown in Figure l. The insert 12 shown in Figure 4 has, in addition to an aperture 11 for the piston pin, two rows of circular apertures, the bottom row of which is formed of comparatively large perforations 13 whilst immediately above it a row of small perforations 14 is arranged.

In the case of the insert 16 shown in Figure 5, two rows of rectangular perforations 17 and 18 are provided at the lower edge.

Figures 6 and 7 show the cross-section of alternative insert 19 and 20 respectively. In the case of the insert 19, the cross-section is slightly wedge-shaped, and, in the case of the insert 20, the cross-section is stepped. With this shaping of the inserts, eiects are obtained which are similar to those obtained by the perforations shown in Figures 1 to 5.

Figure 8 shows an alternative method of producing the same eiect by constructing a piston so that the skirt wall 22 is of wedge-shaped cross-section in the area enclosing a rectangular cross-section 21. In this case, inserts 21 can be employed which have the same resistance to bending at all positions. However, the construction of the skirt shown in Figure 8 may also be combined with the inserts shown in any of the earlier figures.

I claim:

l. A light-metal piston, for internal combustion engines, compressors and the like, of the type having a skirt which is partly separated from the piston head by transverse slots or perforations and which is reinforced by curved metallic inserts made of a metal which has a thermal coefficient of expansion less than that of the light-metal material of the piston, said piston having wrist pin apertures and said inserts having intercontacting areas with the skirt and being arranged to provide a bimetallic eifect of bending stresses with changes of temperature, characterized in that the inserts are made of metal strip and that the inserts and the piston skirt are so constructed and proportioned that the bimetallic effect of the inserts on the piston skirt is less for the lower portion of the intercontacting areas of the skirt and the inserts than it is for the top portion of the said areas and decreases progressively at least from the top to the bottom of the lower portion of said areas, each insert having an aperture therethrough in register with the wrist pin aperture, and each insert having a substantially straight top edge and extending continuously along its said top edge above the wrist pin aperture, whereby the bimetallice eiect above the level of the wrist pin is provided by an uninterrupted length of metal forming the top of the insert.

2. A light-metal piston according to claim 1 wherein the resistance to bending of the inserts decreases from the top to the bottom of their mutually contacting areas with the skirt.

3. A light-metal piston according to claim l wherein the inserts are weakened by at least one row of perforations near their bottom edges.

4. A iight-metal piston according to claim 1 wherein the inserts are of wedge-shaped section, tapering from their top edges toward their bottom edges.

5. A light-metal piston according to claim 1 wherein the inserts decrease in thickness from their top edges towards their bottom edges in definitely defined steps.

6. A light-metal piston according to claim 1 wherein the skirt of the piston increases in thickness between the top and the bottom of the area over which it is in contact with the inserts.

References Cited in the file of this patent UNlTED STATES PATENTS 1,681,621 Nelson Aug. 21, 1928 2,062,625 Norton Dec. 1, 1936 2,177,574 Malina Oct. 24, 1939 2,190,207 Dillon Feb. 13, 1940 2,238,087 Bowser et al Apr. 15, 1941 2,271,024 Nicolle et al Jan. 27, 1942. 

